"How long will you be in rehab?" he asked. She answered "three months."

"How many times have you been in rehab?" he added.

"Several," she replied.

"And how will this time be different? What are they rehabbing, first of all? What is on their list? What are they going to work on when you walk through the door?" Letterman asked, while the audience laughed.

"I'm the happiest when I'm working and the healthiest," Lohan said. "And I think this an opportunity for me to, you know, focus on what I love in life. And I don't think it's a bad thing. I think it's a blessing … and not a curse."

When he asked her whether she had addiction problems, Lohan replied: "Now you sound like Dr. Phil."

Lohan will spend three months of rehab, starting on May 2. The stint was part of a March plea deal that allowed her to avoid jail and avert a trial in her alleged lying-to-police case. In addition to rehab, she will do 30 days of community labor.

The "Liz & Dick" star also agreed to pay an undisclosed fine and attend 18 months of psychological therapy.

Lohan's rehab will be "locked," which means she will not be able to go home once she enters the program. If she completes all the terms of the deal, a 180-day jail sentence will be waived.

The deal was struck just as Lohan's trial on charges that she lied to police about whether she was behind the wheel in a June car crash that briefly sent her to the hospital was set to begin. The 26-year-old actress had pleaded not guilty to three misdemeanor charges filed after the accident: reckless driving, lying to police and obstructing officers from performing their duties.